Putting the 'Don't Trust What a Prospect Does in September' Theory to the Test

When evaluating players, statistically speaking, there are three pretty clear no-no's: Don't trust September numbers. Don't use small samples. And don't rely on young players.

Let's ignore all three!

All of the above caveats exist—and for good reason—but since September is bearing down on this season, it's as good a time as any to explore a very specific niche: how prospects who debut in September perform, both that month and in the ensuing season.

Going back over the past five seasons (2008-12), the names below include all prospects who made their major league debuts during September roster expansion and who also were on Baseball America's Top 100 Prospects list either entering that season or the following one.

It's challenging to pick proper statistics to help measure performance and production over, say, 50 plate appearances or 20 innings pitched in a single month, especially among players who are about 20 years old, give or take a year or two.

For pitchers, the statistics of choice are:

Earned Run Average Minus (ERA-): 100 is league average, with each point above/below representing a percent above/below league average, and lower is better

Fielding Independent Pitching Minus (FIP-): 100 is league average, with each point above/below representing a percent above/below league average, and lower is better

For hitters, they are:

Weighted On-Base Average (wOBA): .320 is league average, and higher is better

Weighted Runs Created Plus (wRC+): 60 is league average, and higher is better

The reason these stats are being used is because they are weighted rate metrics, which makes it easier to compare September performance relative to the rest of the league, particularly when the sample sizes are already tiny.

Once the September numbers are gathered for each prospect, we'll take a look at their performance the next season in those same metrics, while adding in Wins Above Replacement (WAR). This way, we'll get a sense of whether there's any carry-over—good or bad—from one season's final month into the ensuing year.

September 2008 Debuts

Seven top prospects debuted, three pitchers and four position players:

Pitcher Prospects Who Debuted in September 2008

PROSPECT

DEBUT

SEPTEMBER STATS

2009 STATS

DATE

ERA-/FIP- (IP)

ERA-/FIP-/WAR (IP)

Jonathon Niese, LHP, Mets

Sep. 2

169/122 (14.0)

104/79/0.5 (25.2)

David Price, LHP, Rays

Sep. 14

46/81 (14.0)

104/109/1.3 (128.1)

James McDonald, RHP, Dodgers

Sep. 17

0/70 (6.0)

98/109/0.0 (63.0)

FanGraphs

As you can see, it's tough to get too excited about many young players' September numbers because they just don't get much playing time. To that point, Jonathon Niese and David Price managed all of 14 innings, while James McDonald notched only six.

Doug Benc/Getty Images

Price, you'll recall, was used primarily out of the bullpen upon his call-up, which was meant to conserve his innings a bit while preparing him for his postseason role. The left-hander was the most impactful of this trio of arms in September of 2008, and he followed that up by being the best of the bunch in the 2009 season. Even if Price disappointed somewhat—his 4.42 ERA was the highest—he did tally 1.3 WAR due to his 128.1 innings.

Position Prospects Who Debuted in September 2008

PROSPECT

DEBUT

SEPTEMBER STATS

2009 STATS

DATE

wOBA/wRC+ (PA)

wOBA/wRC+/WAR (PA)

Matt Antonelli, 2B, Padres

Sep. 1

.262/60 (65)

N/A

Dexter Fowler, OF, Rockies

Sep. 2

.160/-23 (27)

.342/97/0.6 (518)

Mat Gamel, 1B/3B, Brewers

Sep. 3

.630/295 (2)

.331/100/0.3 (148)

Alcides Escobar, SS, Brewers

Sep. 3

.448/176 (4)

.312/87/0.3 (134)

FanGraphs

Not exactly a big September for hitters here. The two Brewers, Mat Gamel and Alcides Escobar, weren't half bad in their first looks, but they didn't do much the following season. Gamel has never been able to stay healthy since debuting, while Milwaukee sent Escobar to the Royals as part of the Zack Greinke trade after the 2010 season (and probably doesn't regret it).

Dexter Fowler became a solid player for the Rockies in year two, but he's never really taken that step forward many expected. As for Matt Antonelli, he batted injuries and sub-par performance and never made it back to the bigs.

Doug Pensinger/Getty Images

September 2009 Debuts

Seven top prospects debuted, four pitchers and three position players:

Pitcher Prospects Who Debuted in September 2009

PROSPECT

DEBUT

SEPTEMBER STATS

2010 STATS

DATE

ERA-/FIP- (IP)

ERA-/FIP-/WAR (IP)

Carlos Carrasco, RHP, Indians

Sep. 1

201/162 (22.1)

95/102/0.4 (44.2)

Daniel Hudson, RHP, White Sox

Sep. 4

73/116 (18.2)

57/80/2.4 (95.1)

Wade Davis, RHP, Rays

Sep. 6

87/69 (36.1)

103/120/0.7 (168)

Madison Bumgarner, LHP, Giants

Sep. 8

44/111 (10.0)

78/96/1.7 (111.0)

FanGraphs

Rick Yeatts/Getty Images

What's interesting here is that Wade Davis saw the most action at the end of 2009 (36.1 IP) and was rather good (3.72 ERA), then when 2010 rolled around, the Rays right-hander once again logged the most frames (168.0)—but was easily the least effective (4.07 ERA) of this quartet.

For what it's worth, Daniel Hudson was traded to the Arizona Diamondbacks in July 2010 and tallied just shy of 80 innings with his new team in the other league.

Position Prospects Who Debuted in September 2009

PROSPECT

DEBUT

SEPTEMBER STATS

2010 STATS

DATE

wOBA/wRC+ (PA)

wOBA/wRC+/WAR (PA)

Tyler Flowers, C, White Sox

Sep. 3

.295/71 (20)

.247/43/-0.1 (15)

Buster Posey, C, Giants

Sep. 11

.105/-51 (17)

.371/134/3.9 (443)

Adam Moore, C, Mariners

Sep. 17

.279/67 (24)

.227/40/-1.1 (218)

FanGraphs

A backstop bonanza! All three top prospects on the hitter side were catchers, but none of them got much of a look in September of 2009. Of the trio, only Buster Posey—who won the NL Rookie of the Year and the World Series with the Giants after coming up in late May 2010—went on to any major league success right away (or ever).

Stephen Dunn/Getty Images

September 2010 Debuts

Nine top prospects debuted, two pitchers and seven position players:

Pitcher Prospects Who Debuted in September 2010

PROSPECT

DEBUT

SEPTEMBER STATS

2011 STATS

DATE

ERA-/FIP- (IP)

ERA-/FIP-/WAR (IP)

Jake McGee, LHP, Rays

Sep. 14

46/62 (5.0)

116/122/-0.2 (28.0)

Kyle Drabek, RHP, Blue Jays

Sep. 15

114/97 (17.0)

145/133/-0.1 (78.2)

FanGraphs

Jake McGee had been brought along as a starter in the minors, but the Rays put him in the pen once he graduated to the bigs, in part because they had other arms for the five-man and partly because he underwent Tommy John surgery in the 2008 season. The southpaw has been an effective reliever ever since.

Kyle Drabek, meanwhile, was supposed to be a member of the Blue Jays rotation in 2011 after his 17-inning indoctrination in September of 2010. Alas, the righty was pretty terrible (6.06 ERA) and eventually went in for ligament replacement surgery, too.

Position Prospects Who Debuted in September 2010

PROSPECT

DEBUT

SEPTEMBER STATS

2011 STATS

DATE

wOBA/wRC+ (PA)

wOBA/wRC+/WAR (PA)

Desmond Jennings, OF, Rays

Sep. 1

.283/77 (24)

.355/128/2.3 (287)

Freddie Freeman, 1B, Braves

Sep. 1

.214/25 (24)

.348/120/0.7 (635)

Danny Espinosa, 2B, Nationals

Sep. 1

.311/90 (112)

.324/104/3.2 (658)

Yonder Alonso, 1B, Reds

Sep. 1

.211/21 (29)

.411/160/0.6 (98)

Lars Anderson, 1B, Red Sox

Sep. 6

.269/57 (43)

.000/-100/-0.2 (5)

Brent Morel, 3B, White Sox

Sep. 7

.298/78 (70)

.289/74/0.2 (444)

Hank Conger, C, Angels

Sep. 11

.267/64 (34)

.281/77/0.3 (197)

FanGraphs

This is an intriguing group, mainly because Freddie Freeman, Danny Espinosa and Brent Morel all got cups of joe in preparation for what were more or less full-time gigs for their teams by the following Opening Day. Freeman and Espinosa went on to have nice rookie years, but Morel didn't pan out.

Josh D. Weiss-USA TODAY Sports

Desmond Jennings and Yonder Alonso were pretty much ready to step in from the get-go in 2011 too, but they both had to wait until late July before getting recalled. Jennings fell victim to the Rays' preference to keep their promising prospects in the minors as long as possible, and Alonso was blocked at first base in Cincinnati by the reigning NL MVP in Joey Votto (and subsequently traded to San Diego as part of the Mat Latos deal in December 2011).

As for the final two, Lars Anderson soon turned into a prospect bust, and while Hank Conger never received a legitimate shot with the Angels, it could be argued he never really deserved one, either.

September 2011 Debuts

A total of 14 top prospects debuted, seven pitchers and seven position players:

Pitcher Prospects Who Debuted in September 2011

PROSPECT

DEBUT

SEPTEMBER STATS

2012 STATS

DATE

ERA-/FIP- (IP)

ERA-/FIP-/WAR (IP)

Addison Reed, RHP, White Sox

Sep. 4

87/46 (7.1)

111/84/0.8 (55.0)

Brad Peacock, RHP, Nationals

Sep. 6

20/101 (12.0)

N/A

Drew Pomeranz, LHP, Rockies

Sep. 11

124/59 (18.1)

109/107/1.0 (96.2)

Matt Moore, LHP, Rays

Sep. 14

75/57 (9.1)

98/100/2.5 (177.1)

Andrew Brackman, RHP, Yankees

Sep. 22

0/165 (2.1)

N/A

Dellin Betances, RHP, Yankees

Sep. 22

161/226 (2.2)

N/A

Jarrod Parker, RHP, Diamondbacks

Sep. 27

0/80 (5.2)

88/85/3.5 (181.1)

FanGraphs

Drew Pomeranz got the biggest audition after coming to the Rockies a few months earlier in the trade involving Ubaldo Jimenez, and despite a 5.40 ERA, he wasn't wholly ineffective (2 W, 2.59 FIP). The lefty then had trouble adjusting to the majors—and Coors Field—in 2012, as he put up a 4.93 ERA in nearly 100 frames.

The real success stories of this batch were Matt Moore, Jarrod Parker and Addison Reed, all of whom went on to log plenty of work as rookies in 2012.

Following a dynamite one-start cameo late in the 2011 regular season and an even better showing in Game 1 of the ALDS against the Rangers that October (7 IP, 0 R, 2 H, 6:2 K:BB), a ton was expected of Moore heading into the next year. While the southpaw was solid (11 W, 3.81 ERA), Parker was actually better (13 W, 3.47 ERA) for the Athletics, who acquired him as part of the trade involving Trevor Cahill over the winter.

Reed, on the other hand, became a late-inning weapon for the White Sox and has since turned into a quality closer.

Andrew Brackman and Dellin Betances, a pair of Yankees prospects who were always inconsistent, didn't pitch an inning in the majors in 2012. Nor did Brad Peacock, who made two starts for Washington in 2011 and then was dealt to the Athletics as part of the package of prospects for Gio Gonzalez.

Position Prospects Who Debuted in September 2011

PROSPECT

DEBUT

SEPTEMBER STATS

2012 STATS

DATE

wOBA/wRC+ (PA)

wOBA/wRC+/WAR (PA)

Jesus Montero, C, Yankees

Sep. 1

.422/166 (69)

.295/90/-0.4 (553)

Leonys Martin, OF, Rangers

Sep. 2

.381/135 (8)

.257/52/-0.4 (52)

Devin Mesoraco, C, Reds

Sep. 3

.249/49 (53)

.275/67/0.0 (184)

Wilin Rosario, C, Rockies

Sep. 6

.292/67 (57)

.356/110/1.3 (426)

Matt Dominguez, 3B, Marlins

Sep. 6

.280/71 (48)

.335/110/0.3 (113)

Joe Benson, OF, Twins

Sep. 6

.273/67 (74)

N/A

Austin Romine, C, Yankees

Sep. 11

.168/-6 (20)

N/A

FanGraphs

More catchers! Four more, in fact. And of this quartet of backstops, Jesus Montero was supposed to be the guy, especially after how he looked in his initial action with the Yankees that September. Alas, he was traded only months later to the Mariners for Michael Pineda. The deal hasn't worked out for either side.

Michael L. Stein-USA TODAY Sports

The catcher who turned out to be the top guy the next season was actually Wilin Rosario (.270, 28 HR), as Devin Mesoraco fell into a backup role in Cincinnati, while Austin Romine, the other Yankees catcher here, battled a back injury that kept him out most of 2012.

None of the non-catchers amounted to much in their second go-rounds. Matt Dominguez was traded in June of last season to the Houston Astros for Carlos Lee, and he's since become a serviceable starter with a good glove and some power (19 HR), but Joe Benson remained injury-prone, struggled like crazy in the minors and hasn't been back since.

On the plus side, though, Leonys Martin has developed into a quality player for the Rangers, but it took until this season, so he doesn't get credit for the follow-up campaign.

September 2012 Debuts

Seven top prospects debuted, four pitchers and three position players:

Pitcher Prospects Who Debuted in September 2012

PROSPECT

DEBUT

SEPTEMBER STATS

2013 STATS

DATE

ERA-/FIP- (IP)

ERA-/FIP-/WAR (IP)

Shelby Miller, RHP, Cardinals

Sep. 5

34/49 (13.2)

80/91/2.3 (139.2)

Tony Cingrani, LHP, Reds

Sep. 9

45/83 (5.0)

72/94/1.5 (97.2)

Jake Odorizzi, RHP, Royals

Sep. 23

118/129 (7.1)

157/122/0.1 (18.0)

Dylan Bundy, RHP, Orioles

Sep. 23

0/116 (1.2)

N/A

FanGraphs

None of these four arms got much of a taste last September, but Shelby Miller looked mighty impressive in his 13.2 frames (9 H, 2 R, 16:4 K:BB) and even earned a spot on the Cardinals' postseason roster—all of which set up his breakout Rookie of the Year-caliber 2013 campaign (12-8, 2.90, 9.7 K/9 in 25 starts).

Your browser does not support iframes.

Cingrani, meanwhile, broke in a year ago in a relief role because the Reds had a full rotation. When they needed him after Johnny Cueto went down early on this year, though, the lefty was ready to take on a starting gig, and he's looked great so far (6-3, 2.76 ERA, 10.3 K/9 in 16 starts).

As for Jake Odorizzi and Dylan Bundy, they haven't been as lucky. The former likely would have spent more of 2013 in the majors had he not been a part of the Rays' bounty (along with Wil Myers) in the James Shields deal. The latter, who entered the season as the top pitching prospect in the sport, didn't throw a single pitch that counted due to arm problems that eventually required Tommy John surgery.

Position Prospects Who Debuted in September 2012

PROSPECT

DEBUT

SEPTEMBER STATS

2013 STATS

DATE

wOBA/wRC+ (PA)

wOBA/wRC+/WAR (PA)

Jurickson Profar, 2B/SS, Rangers

Sep. 2

.269/60 (17)

.291/74/-0.4 (270)

Adam Eaton, OF, Diamondbacks

Sep. 4

.355/118 (103)

.322/99/-0.1 (154)

Didi Gregorius, SS, Reds

Sep. 5

.265/60 (21)

.312/92/1.2 (360)

FanGraphs

While Jurickson Profar homered in his first-ever big league at-bat, it was Adam Eaton who looked most major league-ready heading into 2013. Alas, an elbow injury suffered late in spring training derailed what many felt could have been a Rookie of the Year-caliber campaign, and he only rejoined the Diamondbacks in July.

Your browser does not support iframes.

Profar, meanwhile, entered the year as the consensus top prospect in baseball, but he's been bounced around the diamond and the lineup by the Rangers, who don't have an easy fit for the 20-year-old phenom. His future remains extremely bright, but Profar's rookie season hasn't lived up to the hype.

That leaves Didi Gregorius, who's been barely competent at the plate but a strong defender at a premium position for the Diamondbacks, to whom the shortstop was traded by the Reds in the offseason.

Conclusions

So what did we learn? Well, in many cases of top prospects who debut in September, their actual first taste of the majors is really too small to evaluate on any legitimate level. Really, what are we supposed to make of a few dozen at-bats or a couple of starts?

As for the following season, though, there's more to work with. To wit, of the 44 debuts discussed above, 16 could be counted as success stories as far as only the ensuing year specifically. Of the 20 pitchers, eight fit that mold: Price, Hudson, Madison Bumgarner, Reed, Moore, Parker, Miller and Cingrani. And the same number applies to the 24 position players: Fowler, Posey, Jennings, Freeman, Espinosa, Alonso, Rosario and Gregorius.

Christian Petersen/Getty Images

Based on that, it seems like it's a bit easier for pitchers to debut in September of one year and then make an immediate impact the following season. Shelby Miller is the best example of this in 2013. The best instance of this among position players? Well, Posey—duh.

Another angle here is that of the 44 debuting players, only seven didn't play in The Show the next year. And of those, only four failed to return to the majors at all, meaning big prospects who get there in September tend to stick around. (Note: This is assuming, of course, that Bundy recovers from his surgery and regains his status as a top prospect while working his way back to Baltimore.)

One final thought. Of the prospects highlighted, nine were traded shortly after making their debuts and before exhausting their rookie status (130 at-bats, 50 innings pitched). In other words, don't be surprised to see a few of the top young players who get the call for the first time this September to be wearing different uniforms next season.

In addition to giving elite prospects their first taste of the major leagues, September also can serve another purpose—a platform to showcase top young talent.